Chassis structure for a forest machine

ABSTRACT

The invention relates to a chassis structure for a working machine moving on a terrain, the chassis structure ( 40, 60 ) comprising a pair of front wheels and a pair of rear wheels supporting the frame ( 41  to  42, 61  to  62 ), and a controlled frame joint ( 63 ) between the frame parts ( 41, 42, 61, 62 ), when the frame is composed of two frame parts. The chassis structure ( 40, 60 ) further comprises a suspension ( 1   a ) controlled by actuators and arranged to move each wheel in a level transverse in relation to the frame. Each wheel is mounted rotatably and to turn by means of the actuators to the suspension, and each wheel is usually provided with hub motor means for drive transmission. The chassis structure ( 40, 60 ) can further comprise a pair of middle wheels for a second frame part ( 42, 62 ). In a method related to the chassis structure ( 40, 60 ) the forest machine can also be turned in place.

[0001] The invention relates to a chassis structure for a forest machinemoving on a terrain in accordance with the preamble of the claim 1.

[0002] For harvesting, forest machines are known which move on a terrainby means of wheels. These include a harvester in which a harvestingdevice is provided at the end of a boom assembly, a so-called harvesterhead for cutting and felling a tree stem and sawing it to pieces ofdesired length. The sawed tree stems are collected by a loadingapparatus, i.e. a forwarder equipped with a grapple, and transported inthe load space.

[0003] Publication WO 89/00928 discloses a working machine thatcomprises two frame parts connected to each other by means of a joint.The joint is used for controlling the direction of motion of the workmachine. The working machine travels along a curved line by foldingframe parts, wherein the centre of turn and the pivoting axle are alwaysat an intersection in which the directions that are perpendicularrelative to the frame parts intersect, projected to the same horizontalplane. Said direction is at the same time the direction of the mutualrotation axis of each pair of wheels. The wheels are supported bysuspension arms of a parallelogram mechanism, said arms keeping also awheel vertical.

[0004] Forest machines may comprise a swivel base mounted on the frameto swivel around a vertical axis and equipped with a cabin and, by itsside, a boom assembly provided with a harvester head at its end. In amanner known as such, the boom assembly can also be mounted in front ofthe cabin or on a separate swivel base apart from the cabin.

[0005] The application publication WO 92/10390 discloses a harvestercomprising two frame parts and controlled by folding the frame parts.The wheels are supported by a pendulum arm, wherein the rotation axes ofeach pair of wheels do share the same direction, but are not alwayscoincident, as the height of the terrain differs at different sides ofthe working machine. Thus, the rotation axes intersect even in fourdifferent locations when the frame is folded. Consequently, the actualswivel centre is vague and causes, in view of some of the wheels, alsodiagonal sliding in curves, which damages the terrain. Even under thesecircumstances the centre is always perpendicular in relation to eachframe part. In prior art forwarders, the second frame part is providedwith a load space, wherein the tree trunks are collected from theterrain by means of a rotatably attached boom assembly at the front partof this frame part and its grapple. The second frame part is equippedwith a cabin and a prime mover for the working machine.

[0006] Application publication WO 99/710221 discloses a forwardercomprising three frame parts that are folded when driving in a curve, sothat the rotation axes of all pairs of wheel would share a mutualintersection point. Forces directed to the frame joints are significant,particularly at the beginning of the curve, wherein the turning radiusmust be gradually diminished when moving, if an aim is to prevent thepair of front wheels from advancing diagonally. The wheels are also heresuspended by means of swaying arms, which causes vagueness in the swivelcentre as well as gliding. To increase the capacity of weight the loadspace is supported by two pairs of wheels, but in this case the loadspace must be supported on the frame parts by joints, because straighttree stems do not bend along with the frame parts. The structure iscomplex and when loaded it can even prevent the frame part from foldingor at least increase the folding power needed in the frame joint.

[0007] The application publication DE 19822809 A1 discloses aload-carrying vehicle comprising two two-wheel axle-group structures inboth frame parts to increase the capacity of weight. The wheels of theaxle group sway around a common horizontal rotation axis, which isalways perpendicular in relation to the frame part. In particular whendriving in a curve with the frame folded, the rotation axes of wheelsintersect at least at four different locations, wherein the wheels ofsome of the axle groups are always gliding diagonally, particularly whendriving in a steep curve, which will damage the terrain. The axle groupcauses damage to a wider area because, when gliding, the successivewheels of the axle group have different turning radius due to thedistance between the wheels.

[0008] One known forest machine is a so-called combined machine, whichcombines the functions of a harvester and a forwarder. In this case,e.g. the rear frame is provided with a load space and the boom assemblyis mounted rotatably on the front frame. The boom assembly and the cabincan be placed on the same rotating swivel base.

[0009] The boom assembly is provided with a harvester head, which isapplicable also for loading tree trunks, wherein it is provided e.g.with grapples. The advantage is that harvesting and collection of treestems can be performed simultaneously, wherein the need for a pluralityof different machines is reduced, which reduces damage caused to growingstock.

[0010] For example in forest thinning, several growing trees are left bythe side of the logging road used by the working machine, which limitsthe space available for the working machine. To improve the movabilityand to bypass trees, the turning radius should often have to be as smallas possible, which is, however, limited by the stability requirements ofthe machine. Thus, the working machine cannot be folded infinitely,because when curving, the inner wheels come closer to each other and theouter wheels diverge from each other, wherein the centre of gravity canbe positioned adversely. This will increase the risk of falling,particularly if the working machine is additionally tilted or movingwith a load.

[0011] To sum up, known forest machines share a limited ability to moveon a very varying forest terrain and they cause damage to the terrain.Due to structures and balance, the turning radius are limited and spacedemanding, wherein unnecessary damage is caused also to growing stock.Nevertheless, the wheel must be able to move sufficiently in thevertical direction because of unstable forestry terrain and obstacles tobe bypassed.

[0012] A particular problem is how to pass soft and squashy areas of theterrain. Particularly in 6-wheel or larger working machines and in heavyforwarders the successive wheels add to sinking of these spots and atthe same time the swaying of the frame is increased. Simultaneously, theforces folding the frame joints also increase and the load distributionbetween the wheels can vary, even suddenly.

[0013] It is an aim of the present invention to eliminate theabove-mentioned problems by using a new chassis structure in a forestmachine. The chassis structure is suitable for use e.g. in four andsix-wheel harvesters, forwarders and combined machines.

[0014] The invention is suitable particularly for improving movabilityof a three-axle forest machine, e.g. a six-wheel forest machine providedwith a frame joint. The invention is particularly suitable also forfacilitating handling of a four-wheel forest machine.

[0015] The invention is used for eliminating restrictions related toevasive movements and turning movements of prior art forest machines.

[0016] Using the invention, wheels can be readily added to improvebearing capacity, yet without the need to add frame joints or drivingpower. Thus, two or more pairs of wheels can be placed at the front andrear frame.

[0017] To attain this purpose, the chassis structure according to theinvention is primarily characterized in what will be presented in thecharacterizing part of claim 1.

[0018] Using the invention, considerable advantages are achieved. Bycontrolling the direction of each wheel individually, it is possible todrive also to a direction that differs from the longitudinal directionof the frame, wherein it is possible to move diagonally towards side. Inthis case the machine is driven along different wheel paths, or anobstacle can be bypassed to the side. This will bring about considerableadvantage particularly when passing soft areas, wherein successivewheels pass them distinctively at different locations, whereby sinkingis decreased and the above-described problems are avoided. Particularlyat curves it is important to keep the same-side wheels on the same path,so that the horizontal space required by the six-wheel working machinewould be as small as possible and that damage to the terrain would belimited to a narrow area. Successive wheels can now be guided to followseparate paths also in curve drive, wherein the turning radius of eachwheel is set to be different. When driving in a curve, a same commonturning centre can now be attained precisely.

[0019] Particular advantage is achieved in a six-wheel load-carryingvehicle, wherein the load space can be build in a fixed position on therear frame, because it is not necessary for the rear frame to fold incurve drive. In view of the balance, it is also advantageous that thepoints of support, in which the wheel touches the ground, can be keptalmost stationary in relation to each other and the frame, even whendriving in a curve. Thus, the point of support can be kept as central aspossible and far from the wheels placed at the corners of the framepart. At the same time, it is possible to abandon axle structures thathave a limited ability to maintain the frame of the forest machine inthe horizontal direction. There is more freedom to place the wheels,wherein they can be placed even at regular intervals in order to balancethe loads.

[0020] Particular benefit is achieved in foldable forest machines, whichcomprise a frame joint controlled e.g. by means of a folding cylinder.By means of a folding joint all the successive wheels of six-wheelmachines can now be guided to the substantially same path. Thus, e.g.the swivel centres of two pairs of wheel of the rear frame are arrangedto the same point. The wheels of the front frame are directed to thesame point, as long as all the successive wheels on the same side of theforest machine are arranged at one concentric curve of a circle, e.g. byfolding the front frame to the side of the turning centre. In othercases, at least one wheel of the same side moves along a different wheelpath.

[0021] The frame joint of the invention is not used to change thedirection at all, or only at a small scale, wherein its actuators can bemeasured to considerably lower pressures and stresses. The size of theframe joint can be formed lighter and smaller and it can be more readilydesigned in view of durability and other properties, such as foldingand/or swivelling in order to implement locking. Consequently, foldingcan be carried out by driving the rear or front frame to the side byturning the wheels. As a result of the separate control, it is possibleto change the direction of motion of the forest machine even whenstationary, wherein the wheels are turned to a new direction.

[0022] Particular advantage is obtained when each wheel of a four-wheelharvester, which may even lack the frame joint, is provided with a hubmotor, whereby the rotation directions of the wheels can be changed toopposite. Thus, the machine can be rotated even in place, when thefulcrum can be placed in an area limited by the wheels of the machine,e.g. at the centre of weight. Thus, the turning point of each wheel mustbe sufficient, which can be more readily implemented in a shortharvester. This is beneficial particularly when rounding sharp corners,wherein the working machine is first driven directly to the corner,turned in place, e.g. 90° and driven straight out of the corner. Thus,the necessary space is defined only by the dimensions of the forestmachine itself, not by the turning radius achieved.

[0023] As a result of separate suspension, it is also possible to keepthe load balanced between different wheels, to keep the loaddistribution as desired, and to keep the frame at a desired position invarious situations. Thus, it is possible to measure e.g. the loadpressure of the wheel-suspension actuator by pressure gauge means, theposition of the suspension by position gauge means arranged at itsjoints, and thus also the position of the frame. Based on these, themutual position of the suspensions are controlled on the basis of thesein order to increase or decrease the load of some of the actuators, ifnecessary even without changing the position of the frame. In some casesit is also advantageous to arrange the wheelbase of the rear frame to beadjustable in a six-wheel forest machine. By shortening the wheelbase,steady load can be maintained when surmounting steeper obstacles, evenif the maximum vertical movement of the wheel suspension was the same asin the standard wheelbase. Another advantage is that the turning radiuscan be diminished using the same maximum angles of inclination or,alternatively, the maximum angle of the suspension can be diminishedwithout decreasing the turning radius.

[0024] The chassis structure of the invention can provide a veryversatile control of the wheels under coordination of the control systemof the forest machine and in accordance with the selections of thedriver. The control system, in turn, controls the control means known assuch of the actuators, which in turn control the position and movementof the actuators e.g. by controlling their pressure levels and volumeflows. A working machine is generally provided with a pressure mediumarrangement known as such, in which hydraulic fluid pressured by a pumpis used in energy transmission and led to the actuators and also to thepower transmission system of the wheels. Thus, it is possible to commandthe control system to arrange wheels e.g. to a state in which the forestmachine rotates in place, wherein the control system determines theangles of inclination of the wheels and selects the directions ofrotation in accordance with the desired turning direction. In the secondstate, normal straight drive and curve drive are followed either usingthe same path or different paths, wherein the control system also takescare of folding and/or controlling the wheels along the desired path.The driver takes care e.g. only of the travel direction of the frontwheels and the steepness of the curve, wherein the control system, inturn, takes care of controlling the middle and rearward wheels.

[0025] In the following, the invention will be described in more detailby using as examples advantageous embodiments of the invention withreference to the appended drawings, in which

[0026]FIG. 1 shows the structure of a joint parallelogram of thesuspension in a reduced view and the mounting of a cylinder seen fromthe direction of the rotation axis,

[0027]FIG. 2 shows a module according to an advantageous embodiment of achassis structure of the invention in a perspective view in the middleposition of the wheels,

[0028]FIG. 3 shows a perspective view of a load-carrying vehicleapplying the module of FIG. 2, when a forwarder is diagonally moving tothe side, and

[0029]FIG. 4 shows a top view of a chassis structure according to apreferred embodiment of the invention behaving in curve drive,

[0030]FIG. 5 shows a top view of a chassis structure according to apreferred embodiment of the invention behaving when driving the samepath,

[0031]FIG. 6 shows a top view of turning a chassis structure accordingto an advantageous embodiment of the invention, and

[0032]FIG. 7 shows a perspective view of a frame joint applied in aload-carrying vehicle and chassis structures according to FIG. 3.

[0033]FIGS. 1 and 2 illustrate an arrangement of a chassis structureaccording to the invention for suspension of wheels. FIG. 1 shows asimplified joint parallelogram and various arms thereof in the middleposition. FIG. 1 shows also the positions of suspension arms 6 a and 7 aas well as a cylinder actuator 12 a controlling the position of amounting arm 5 a in the vertical direction. A wheel is mounted to themounting arm 5 a as shown in FIG. 2. For the sake of comparison, priorart placement of a cylinder is illustrated with a broken line C. If thesuspension arms 6 a, 7 a of the joint parallelogram are parallel andequal in length, the vertical position of the mounting arm 5 a andthereby the vertical position of the wheel 2 a are changed as little aspossible in the different positions of the joint parallelogram.Normally, the arms 6 a, 7 a are unequal at least in length.

[0034] The suspension of wheels according to prior art is problematicwhen there is a need to limit the dimensions of the working machine,particularly its width and height, without affecting the capability ofthe forest machine to proceed on the terrain. The cylinder C is normallymounted at a point where the piston stroke becomes very long for movingthe wheel from the upper position to the lower position. For thisreason, the arms also become long, to fit the cylinder C in between,both short and long, which affects particularly the width of the workingmachine, and it is particularly difficult to make the long suspensionarms steadfast. In addition, when there is a need to reduce the width ofthe working machine, the moment arm of the cylinder C must be reduced,wherein the play between the upper position and the lower position isreduced.

[0035] By placing the cylinder 12 a, in accordance with FIG. 1,vertically and mounting its one end 13 a preferably to the lowersuspension arm 6 a and its other 14 a to the frame part 1, it ispossible to reduce the cylinder 12 a stroke in relation to prior art.However, the moment arm of the cylinder 12 a can be kept substantiallyequal relative the prior art, but the mounting point 13 a in the lowersuspension arm 6 a can be shifted closer to the frame 1. Thus, therewill be more space for other structures at the outermost end 8 a of thesuspension arm 6 a. Due to shorter cylinder stroke the vertical cylinder12 a does not harmfully increase the height of the structure,particularly when it is mounted on the lower support arm 6 a. Underneaththe support arm 6 a there is also space for lugs 6 c, which can be usedfor lowering the joint 13 a. As the mounting point 13 a is placed in thesuspension arm 6 a and not in its joint 8 a, a change in the length ofthe suspension arm 6 a will not necessarily affect the structure of thecylinder 12 a, the levels of pressure used, or particularly the momentarm. With the vertical position, also the moment arm formed by thesuspension arms 6 a, 7 a in different positions will be changed less inthe operating range desired when the wheel 2 a moves up and down, whichmakes it easier to regulate the pressure level.

[0036] With reference to FIG. 2, the chassis structure comprises a framepart 1 placed on a vertical central line CL of the working machine andforming a part of a frame of a forest machine intended to move on aterrain, such as a harvester, a forwarder or a combined machine. Theworking machine comprises a foldable front and rear frame part, whichmove by means of one or more pairs of wheels. The wheels are connectedto the frame part by means of a suspension as described. Placed on theframe is also a prime mover for the working machine, typically a dieselengine. The working machine is typically equipped with a hydrostatictransmission system.

[0037] A boom assembly is placed on top of a front frame 61 of a chassisstructure 60 of a foldable four-wheel harvester in accordance with FIG.6. The boom assembly is arranged for example in front of a cabin, andthe end of the boom assembly can be connected to a harvester head. Thecabin and the boom assembly can be located on the same or differentswivel bases, which swivel around vertical axes. They can also belocated in different frame parts. The cabin can also be stationary or atleast tiltable relative the frame or to the side. The motor is placed inthe frame part further behind 62. The frame parts 61, 62 are connectedto each other by the frame joint 63, which allows the turning of theframes with respect to each other around a vertical axis. If thesuspension limits the turning of a wheel, a foldable middle joint can beused to reduce the turning radius, wherein manoeuvrebility is improved.In the absence of the joint 63, the frame parts 61, 62 constitute auniform frame. If the suspension limits the lifting and lowering of thewheel, the swivelling middle joint can be used to increase the verticalmovement of the wheel in order to pass obstacles. Each frame part has atleast one pair of wheels 64 a, 64 b and 65 a, 65 b having suspensionscorresponding to FIG. 2.

[0038] A load space is arranged on top of the rearward frame part 42 ofthe chassis structure 40 of the foldable six-wheel load-carrying vehicleor combination machine in accordance with FIGS. 4 and 5 for transportingtimber, such as delimbed and cut tree stems. The motor is placed in thefront frame part, in front of or behind the cabin.

[0039] The boom assembly of the forwarder is connected in a swivelledmanner to the front part of the rear frame 42. In the combinationmachine the boom assembly is provided on top of the front frame 41, forexample in the rear part thereof. The cabin of the front frame 41 andthe boom assembly can be placed on the same or different swivel bases,swivelling around vertical axes. The frame parts 41, 42 are connected toeach other by the frame joint 43, which at least allows the turning ofthe frame parts with respect to each other around a vertical axis, andadvantageously allows them also to turn with respect to each otheraround an axis X2 that is parallel to the longitudinal direction of theworking machine. The rear frame 42 comprises at least two pairs ofwheels 45 a, 45 b and 46 a, 46 b and the front frame 41 comprises atleast one pair of wheels 44 a, 44 b, having suspensions corresponding tothat in FIG. 2.

[0040]FIG. 2 comprises a chassis structure further equipped with twosubstantially U-shaped bunks 15 a, 15 b, which are mounted on the framepart 1, wherein it forms a part of the rear frame 42 illustrated inFIGS. 3 and 4. A corresponding frame part 1 with suspensions and wheelsbut without bunks, i.e. a module M, can constitute a part of the frontframe part 41 or the frame part 61, 62 of FIG. 6. With further referenceto FIG. 2, the frame part 1 is supported by wheels 2 a and 2 b placed onits each side. The wheels 2 a and 2 b are normally pneumatic rubbertyres fitted on a removable rim. Wheels of FIGS. 4, 5 and 6 correspondto these wheels. The rim is attached to the hub of the wheel in aremovable manner. The hub is, in turn, attached in a rotatable andjournalled manner known as such to a non-rotatable hub frame. The hubframe is attached to the suspension through journalled joints 3 a, 3 b,which also support the hub frame and thus also the rotating wheel. Thewheel hub is rotated in a manner known as such by a pressure mediumdriven hub motor arranged to rotate the rim and the wheels and thus togenerate a drive to move the working machine. A corresponding hub motorcan be fitted on all the wheels of the forest machine. On the otherhand, the hub motor is coupled by means of an axis to the wheel hub and,on the other hand, mounted to the hub frame centrally.

[0041] The wheel hub and the hub frame are placed almost entirely insidethe rim and the hub motor is placed partially inside the hub frame. Thewheel is rotated around a substantially horizontal axis Z. Furthermore,the wheel hub is arranged to swivel around a substantially vertical axisY by means of two on top of each other placed joints 3 a, 3 b, whereinthe direction of motion of the working machine can be controlled bysteering the wheel 2 a. The axis Y travels through joints 3 a, 3 b. Thewheel hub is turned by means of actuators, preferably by pressure-mediumdriven cylinders 4 a, 4 b, which are placed on both sides of the axis Y.They are used to generate a moment force that rotates the wheel 2 aaround the axis Y. The actuators 4 a, 4 b are attached by means of thejoint 16 a, 16 b on the one hand to the mounting arm 5 a and on theother hand by means of the joint to the hub frame. Alternatively, forturning the wheel one double-acting cylinder or two single-actingcylinders are sufficient, but the action is non-linear or largercylinders are needed.

[0042] With reference to FIG. 1, the suspension comprises asubstantially vertical mounting arm 5 a where also the joints 3 a and 3b are arranged. The mounting arm 5 a is mounted to the frame 1 by meansof a lower suspension arm 6 a and an upper suspension arm 7 a, which aresubstantially parallel. Each suspension arm 6 a, 7 a is mounted to themounting arm 5 a by means of a joint 8 a or 9 a, the joints allowing therotation of the suspension arm in relation to the mounting arm around asubstantially horizontal axis. Each suspension arm 6 a, 7 a is mountedto the frame 1 by means of a joint 10 a or 11 a, the joints allowing theswivelling of the suspension arm around an axis that is substantiallyhorizontal in relation to the frame. The axis is parallel to thelongitudinal direction of the forest machine. The suspension arms 6 aand 7 a as well as the joints 8 a, 9 a, 10 a and 11 a constitute aparallelogram-like structure, wherein in spite of the swivelling of thesuspension arms, the mounting arm 5 a remains substantially vertical andthe wheel 2 a remains substantially in the same position, elevated to adifferent height. To be exact, the wheel is moving on a level that issubstantially vertical and perpendicular to the longitudinal direction,because only the supporting arms are rotating around the longitudinaldirection. To increase the ground clearance, the joints 3 a, 3 b aremounted close to the lower part of the mounting arm 5 a or on anextension thereof and the supporting arms 6 a, 7 a are mounted close tothe upper part of the mounting arm 5 a.

[0043] In the arrangement of FIG. 2, the suspension arms 6 a, 6 b,placed on each side of the frame part 1, are arranged to swivel aroundthe same axis R1, by means of joints 10 a and 10 b, respectively. Thus,the joints 10 a and 10 b are preferably placed on said centre line CL.The joint 10 b of the suspension 1 b corresponds to the joint 10 a ofthe suspension 1 a, and the suspensions correspond to each other also inother respects. The wheels 2 a and 2 b are preferably spaced by the samedistance from the centre line CL, and the corresponding arms of thedifferent suspensions are preferably equal in length, wherein thesuspensions of the wheels operate in the same way. Correspondingly, thesuspension arms 7 a and 7 b, placed on each side of the frame 1, arearranged to swivel around the same rotation axis R2, by means of joints11 a and 11 b, respectively. The joints 11 a and 11 b are thuspreferably placed on said centre line CL and above the joints 10 a and10 b. It is obvious that the rotation axes of the supporting arms 6 a, 6b can be placed parallelly and separately on side of each other, whereinthe supporting arms 6 a, 6 b can also be placed crosswise with respectto each other. The rotation axes of the supporting arms 7 a, 7 b can beplaced in a corresponding manner.

[0044] The more detailed structure of the joints may vary, but in thestructure of FIG. 2, the joints 10 a and 10 b, as well as the joints 11a and 11 b, are further placed one after the other in the direction ofthe axes R1 and R2. Thus, the suspension arms on different sides of theframe 1, for example the suspension arms 6 a and 6 b, are placed ondifferent lines, wherein they can also be placed crosswise. For movingthe suspension arms 6 a and 6 b, an actuator 12 a operated by apressurized medium, preferably a hydraulic cylinder, is coupled betweenthe frame 1 and the lower suspension arm 6 a, by means of joints 13 aand 14 a. There can be e.g. two support arms 6 a, 6 b one after anotherto stiffen the suspension, wherein they can also be connected by aflange structure, in which in turn the cylinder 12 a and the joint 13 acan be mounted. The cylinder 12 a is mounted above the upper suspensionarm 7 a and substantially in the vertical direction. The cylinder 12 ais mounted between the joints 8 a and 10 a, wherein the cylinder 12 a isplaced in the free space between the frame 1 and the mounting arm 5 a.By means of the actuator 12 a, the suspension 1 a is maintained in adesired position to support the frame 1 at a desired height. By means ofthe actuators 12 a and 12 b of the suspensions 1 a and 1 b, the framepart can also be kept horizontal on an uneven terrain. The frame can bekept in balance, if the wheel 2 b hits a bump on the terrain or if thewheel 2 a hits a pit on the terrain. In the lowermost position of thewheels 2 a, 2 b, the ground clearance can be increased by adjusting thesuspension.

[0045] The frame part 1 and the suspensions 1 a, 1 b of the chassisstructure are arranged to constitute the module M, which can bereproduced and applied in the implementation. The frame part 1 isproduced in a desired length, wherein it can be directly connected toanother corresponding module M to form a multi-wheel frame part of theworking machine. Modules M can also be connected to each other by meansof a varying frame connector. Thus, between the modules M can be placede.g. a telescopic structure, the length of which is changed by means ofcylinders in the longitudinal direction of the frame part. Thus, forexample the wheelbase of the rearward frame of the forest machine can bechanged. Alternatively, there is a guide structure between the modulesM, which is transferred by means of a pressure-medium driven motor. Atthe same time, the load space is mounted by joints on top of the modulesM in a manner that their mutual movement is allowed.

[0046]FIG. 3 illustrates in more detail a load-carrying vehicle 30 inaccordance with an advantageous embodiment of the invention, comprisinga chassis structure corresponding to the one shown in FIG. 4. Itcomprises a two-wheel front frame 41, which is formed of one module M inaccordance with FIG. 2, and a four-wheel rear frame 42, which is formedof two modules M, which are connected to each other by means of theframe joint in accordance with FIG. 7, which can preferably also belocked. The distance between the modules M of the rear frame 42, whichis also the axle spacing, is in this case fixed. The forest machine 30further comprises, fitted in the rear frame 42, a load space 31, atimber shield 32 and a boom assembly 33. The forest machine 30 furthercomprises a cabin 34 fitted onto the swivel base 35 in the rear frame41, and a motor 36.

[0047]FIG. 7 illustrates a swivel joint 17 that is applicable to be usedas the frame joint of a forest machine. The swivel joint 17 constitutesthe frame joint 43, 63 of the working machine 40, 60 according to FIGS.3, 4 and 6. The swivel joint 17 is fitted between the frame parts 41 and42, which frame parts swivel in relation to each other around an axisX2. The frame part 42 is shown in a cut view. The axis X2 is normallyparallel with the longitudinal direction of the working machine. Acircumferential bearing between the parts 41, 42 is usually placedinside the joint 17. It should be noted that the order of frame parts41, 42 can be changed in order to mount the swivel joint 17. The framepart 41 of the working machine is also provided with a joint 18, bymeans of which the joint 17 and the rest of the frame part (not shown inthe figure) are rotated with respect to each other around a verticalaxis Z1. As a matter of fact, the frame part 41 of FIG. 7 constitutes apart of the swivel joint 17 or even a part of the frame part 42, becausethe part 41 is not folded in relation to the part 42, but is folded inrelation to the rest of the frame part. The joint 18 is utilized whenthe working machine is folded. For this purpose, the joint 18 comprisestwo lugs 18 a and 18 b provided at a distance from each other forbearing. The lugs are preferably placed on different sides of the axisX2. The axis Z1 preferably intersects the axis X2. The joint 18 and thecylinders 20, 21 can also be eliminated totally, wherein the frame parts41 and 42 rotate with respect to each other only. This rotation movementcan also be locked and released in a controlled manner by means of alocking device of the swivel joint 17 known as such.

[0048] Lugs 19 a and 19 b are provided symmetrically on different sidesof the axis Z1, at a distance from each other. The first end of a firstrotating cylinder 20 is mounted by means of a joint to the lug 9 a, andthe first end of a second rotating cylinder 21 is mounted by means of ajoint to the lug 19 b. The second ends of the cylinders 20 and 21 aremounted by means of joints to that part of the first frame part 41 whichis mounted to the joint 18, and is not folded. By controlling the linearmovement of the cylinders the frame parts are turned in relation to eachother around the axis Z1. The cylinders are rotated at both ends arounda substantially vertical axis. The joint 18 and the lugs 19 a, 19 b canbe attached directly to the frame part 42, wherein no rotationalmovement is present and the frame parts 41, 42 are only folded inrelation to each other.

[0049] In FIG. 4 the chassis structure 40 is shown in a curve, whereinthe wheels 44 a, 45 a and 46 a have the same turning radius R1, whereinthe joint 43 is folded, wherein also the wheel 44 a has been obtained tothe same curve C1. When the turning point P1 is located perpendicularlyin relation to the frame part 42, halfway between the wheels 45 a, 46 a,at least the wheels 45 b and 46 b can be placed on the same turningradius R2. Due to the dimensioning of the suspension, the wheel 44 bdeviates from the same path only to a small extent. When the turningpoint P1 is located elsewhere than on the axis L1, all the wheels cannotalways be directed to the same path when the frame parts are folded. Thewheels can however be turned perpendicular in relation to the turningpoint, wherein diagonal movement is avoided. Moreover, by folding thechassis structure it is now possible to constantly minimize the distancebetween the paths of different wheels and to direct them to an almostidentical path.

[0050] In one example, the driver will choose mainly the direction oftravel during driving by turning the wheels 44 a, 44 b, wherein thecontrol system takes care of directing the other wheels towards the samepath. Furthermore, it attends to the mutual position of the wheels 44 a,44 b, so that these could share the same turning centre in a curve. Thiscan be achieved by the chassis structure 40 described and illustrated inFIG. 5. When driving straight ahead, the front wheels 44 a, 44 b havefirst bypassed an obstacle T1 by driving along the curves C2, C3,wherein they further share a mutual turning centre P2, which curves arefollowed also by the middle wheels 45 a, 45 b after having reached thatpoint. The rearward wheels 46 a, 46 b continue further directly to thecurves C1, C2, following said curves. If necessary in some situations,the frame joint 43 can be free and unloaded, wherein the foldingcylinders 20, 21 of FIG. 7 are not used, because the wheels take care ofchoosing the direction and, at the same time, the frame parts are alsofolding to the right position automatically. For diagonal driving, theturning angles of all the wheels are set to be the same, as the frameparts extend directly and the frame joint 43 is locked, but with theframe parts folded, the angles of the wheels vary when being directed tothe same direction. When driving a curve, the inner wheel of the pair ofwheels must have a smaller rotation speed than the outer wheel toprevent it from sliding and to prevent damage to the terrain. In orderto provide balanced drive, every wheel is preferably provided with a hubmotor, wherein the wheel can be also actively directed to a desireddirection.

[0051]FIG. 6 shows a special structure of the chassis structure 60 for afour-wheel harvester and a control space in which it can be turned inplace. The turning centre P3 of the turning circle C4 is thus placed onan area limited by the wheels, for example centrally. In this case, theinner sides of the wheels 64 a, 64 b and 65 a, 65 b are turnedperpendicular in view of the centre P3 and towards it, wherein alsotheir direction of rotation is perpendicular. In accordance with FIG. 2,the axis Y is placed also inside the wheel, wherein it is located on theside of the frame, between the wheel and the frame part. Particularlynoteworthy is the crosswise positioning of each pair of wheels, whereinthe wheels of the pair of wheels are turned to opposite directions bythe wheel proportions of A1, A2, A3 and A4. The direction of rotation ofthe hub motor is chosen so that the wheels of each pair of wheels arerotating to opposite directions, and the wheels on the same side of theworking machine, for example 64 a and 64 b, are rotating to the samedirection. The angles A1, A2, A3 and A4 remain small compared to asituation in which the other wheel of the pair of wheels should beturned 180° to the opposite direction, if the wheels of the pair couldnot have a different direction of rotation. Normally, the angles A1, A2are of the same size and angles A3, A4 are of the same size with eachother, when the point P3 is on the line X1. In the figure, the framejoint 63 is locked, which corresponds to the operation of a uniformframe. The angles A1, A2 (and A3, A4) are different from each other whenthe frame joint 63 is folded in an angle that will be taken into accountby the control system in the calculation when setting the wheels. Whenthe turning centre P3 is located elsewhere than in the area limited bythe wheels, the outer sides of the wheels on one side and the innersides of the wheels on the other side are always directed towards theturning centre.

[0052] The invention is not limited solely to the above-presentedembodiment, but it can be modified within the scope of the appendedclaims. The same principles can be implemented also in an eight-wheelworking machine, wherein there exist two successive middle wheel pairssupporting different frame parts. Furthermore, it is obvious thatparticularly in a six-wheel harvester the frame that is generallyfrontward in view of the motion of travel can also be said second framepart.

1. A chassis structure for a forest machine moving on a terrain, whichchassis structure comprises at least: a pair of adjacent front wheelsarranged to support a first frame part placed therebetween, a pair ofadjacent rear wheels arranged to support a second frame part placedtherebetween, and a pair of adjacent middle wheels, which is alsoarranged to support the second frame part, a frame joint arranged tofold the frame parts in relation to each other at least around an axisthat is vertical and transverse in relation to the longitudinaldirection of the working machine, first actuator means to control thefolding, a suspension between each wheel and frame part, wherein thesuspension comprises second actuators arranged to change the position ofthe suspension in relation to the frame and, at the same time, toelevate and descend the wheel, wherein: said suspension is arranged toelevate and descend the wheel with respect to the frame part in a planethat is vertical and transverse in relation to the horizontallongitudinal direction of the working machine, wherein each wheel ismounted in a rotatable and turnable manner to said suspension, whereinsaid suspension is also arranged to turn the wheel in relation to theframe around an axis that is vertical and transverse in relation to saidlongitudinal direction, wherein the suspension further comprises thirdactuators, which have been arranged to turn the wheel with respect tothe suspension and, at the same time, with respect to the frame part,wherein the machine further comprises a control system arranged to guidethe actuators to the correct position at each moment of time during thedrive, wherein, in order to reduce the space used in the horizontallevel during bypassing an obstacle after driving straight ahead, thecontrol system is arranged to control the pair of middle wheels to drivethe same curve which the pair of front wheels is controlled to followfor bypassing the obstacle, after the pair of middle wheels have reachedthe curve by driving straight ahead and during which the front wheelsare already following the curve, and wherein at the same time thefolding between the frame parts is allowed.
 2. A chassis structure asset forth in claim 1, wherein each wheel is fitted with hub motor meansfor transmission.
 3. A chassis structure as set forth in claim 1,wherein the suspension comprises a mounting arm, a lower suspension armand an upper suspension arm, wherein the lower suspension arm is mountedby means of a first joint to the frame part and by means of a secondjoint to the mounting arm, and which upper suspension arm is mounted bymeans of a third joint to the frame part and by means of a fourth jointto the mounting arm, wherein each wheel is mounted by means of a jointto the mounting arm, second actuators have been mounted between thelower support arm and the frame part, and the third actuators have beenmounted to the mounting arm.
 4. A chassis structure according to claim1, wherein the frame joint is also arranged to allow free rotation ofthe frame parts around the longitudinal direction of the working machinein relation to each other and to lock said rotation.
 5. A chassisstructure as set forth in claim 1, wherein each frame part is composedof one or a plurality of identical module structures comprising a framepart, which constitutes a part of said frame part, and two saidsuspensions, which are substantially symmetrically placed on differentsides of said frame part for suspension of one pair of wheels.
 6. Achassis structure as set forth in claim 1, wherein the frame jointcomprises a joint part that is mounted for implementation of a turn tothe first frame part by means of a vertical joint, wherein between saidjoint part and said frame part are mounted first actuators for folding,and for rotation of the bearing between the second frame part and saidjoint part, and for locking the swivelling of the locking apparatus. 7.A chassis structure as set forth in claim 1, wherein the control systemarranged to guide the actuators at each moment of time during the driveto the correct position in order to follow the same or a different pathof the wheels.
 8. A chassis structure as set forth in claim 1, whereinin order to reduce the space used in the horizontal level the controlsystem is arranged during the drive to control the pair of middle wheelsor the pair of rear wheel to the same path which the pair of frontwheels is controlled to follow, wherein at the same time the foldingbetween the frame parts is allowed.
 9. A chassis structure as set forthin claim 1, wherein in order to distribute loadings to different pointsof the terrain, the control system is arranged to control during thedrive the pair of middle wheels and/or the pair of rear wheel todifferent paths that differ from the path used by the of front wheels iscontrolled to follow, wherein at the same time the folding between theframe parts is allowed or blocked.
 10. A chassis structure as set forthin claim 1, wherein the distance between the pair of rear wheels and thepair of middle wheels is arranged to be changed in the longitudinaldirection of the working machine.
 11. A chassis structure as set forthin claim 1, wherein in order to reduce the space used in the horizontallevel during driving a curve the control system is arranged during thedrive to control the pair of middle wheels and the pair of rear wheelsto the same path which the pair of front wheels is controlled to follow,wherein at the same time the folding between the frame parts is allowed.12. A chassis structure as set forth in claim 1, wherein order to reducethe space used in the horizontal level during driving a curve thecontrol system is arranged during the drive to control the pair ofmiddle wheels and the pair of rear wheel to the same path, wherein atthe same time the folding between the frame parts is allowed.
 13. Achassis structure as set forth in claim 11, wherein the turning centreof the curve is located perpendicularly in relation to the second framepart, halfway between the middle wheels and the rear wheels.
 14. Achassis structure as set forth in claim 1, wherein the control system isalso arranged to control the pair of rear wheels to drive the same curveafter having reached the curve by driving straight ahead.